The present invention relates to a method of installing a buoy, particularly, but not exclusively, a subsea production buoy used in deep water hydrocarbon production facilities employing hybrid riser configurations. The invention also provides apparatus for tensioning a subsea production buoy to an anchoring location, particularly, but not exclusively, an anchoring location provided on a subsea foundation.
In deep water production fields, rather than installing a fixed production platform, it is common to anchor a floating production, storage and offloading (FPSO) vessel at a suitable surface location near the field. The produced fluids are recovered from the subsea well(s) to the seabed and then carried along pipelines laid on the seabed to the FPSO. The fluids are processed and stored on the FPSO before being transported, normally by tanker, to an onshore facility for further processing/distribution.
The connection between the pipeline laid on the seabed and the FPSO is typically provided by a steel catenary riser (SCR). The SCR is suspended in the water in axial tension by a subsea buoy tethered to the seabed. With such an arrangement, the SCR extends only from the subsea pipeline to the subsea buoy where it is coupled, through a suitable connection, to a flexible riser. The flexible riser then hangs between the subsea buoy and the FPSO. This connection system is sometimes called a “de-coupled system”. Here the heave motions of the surface vessel are de-coupled from the subsea buoy motions and thus the SCRs hanging from it.
To meet operational requirements, it is important that such subsea buoys are maintained at an appropriate depth and at an appropriate location in the water. This can be problematic due to the large distance between the surface and the foundation to which the buoy is to be anchored.
Another problem is that localised water currents require that the tethers extend from the buoy to the anchoring location at a varying angle. If handled incorrectly, this can cause localised areas of excessive force on the tethers adjacent the connections with the buoy, which can in turn lead to premature failure of the tethers.